1. Field of Technology
This technology relates to a device for controlling the flow of light and radiant heat through selective absorption or reflection of light. The technology has particular, but not exclusive, application in passive or active light-regulating and temperature-regulating films, materials, and devices, especially as a construction material.
2. Description of the Related Art
Switchable mirrors exist which are based on reversible metal hydride and metal lithide chemistry described, for example, in U.S. Pat. No. 7,042,615 to Richardson. These switchable mirrors, which are chemically related to rechargeable batteries, may rely on the physical migration of ions across a barrier under the influence of an electric field and, therefore, have limited switching speeds and cycle lifetimes. In addition, electrically operated “light valves” that combine liquid crystals with one or more reflective polarizers are described, for example, in U.S. Pat. No. 6,486,997 to Bruzzone et al. In these devices, a liquid crystal typically serves as an electrotropic depolarizer, i.e., a means of variably altering or rotating the polarity of the light that passes through it, under the influence of an electric field. Some of these devices can be thought of as switchable mirrors, although they are rarely described that way, since their primary application is in video displays, video projectors, and advanced optics.
Switchable electric light valves that do not require polarizers, but are diffusive forward scatterers or diffusive reflectors, also exist. This is because liquid crystals themselves may act as reflectors (including but not limited to distributed Bragg reflectors or DBRs) with different reflection bands in these applications, with a reflective, diffusive, or forward-scattering mode, and a more transmissive mode. These include the polymer-dispersed liquid crystal (PDLC) display, the cholesteric liquid crystal display (Ch-LCD), the Heilmeier display, and the Guest-Host display. The PDLC is an electrochromic device where the index of refraction of liquid crystal droplets embedded in another material is changed electrically, resulting in more scattering of the light in one mode than another. The Ch-LCD has two stable states, the reflective planar and focal conic texture. The reflective planar structure reflects light if the Bragg reflection condition is met and thus acts as a Bragg reflector for one circular polarization of light, while the reflective focal conic transmits more of the light.
An optical structure called a Guest-Host display commonly utilizes dyes dispersed in a liquid crystal, which absorb more light when in one orientation than in another. The orientation of the dyes is dependent on the orientation of the liquid crystal, which is determined using an electric field created by a voltage, typically applied via transparent conducting layers such as indium tin oxide. Such devices may also utilize one or more polarizers. There are positive and negative dichroic (pleochroic and negative dichroic) dyes, among others, which respectively absorb light along different axes of the molecule.
Polymer-stabilized liquid crystals are created when prepolymers and liquid crystals are mixed and the prepolymer is polymerized, to among other things establish or reinforce the orientation of the liquid crystals. Liquid crystal mixed with prepolymers which are cured in various ways and concentrations has been described in the literature, among other terms, as polymer-stabilized, polymer-networked, polymer-enhanced, and polymer-dispersed, among many other terms. This technology is well described in the prior art as, for example, in U.S. Pat. No. 7,355,668 to Satyendra et al., which discloses polymer-enhanced liquid crystal devices, specifically electrically operated display devices, built with rigid or flexible substrates that include polymer “columns” formed between substrate films through the phase separation of a prepolymer (e.g., Norland NOA77 or 78 optical adhesive) and a liquid crystal (e.g., Merck E7, E48, or E31), under the influence of temperature variations. The prepolymer and liquid crystal are mixed above the clearing point temperature of the LC, and are then cooled below the clearing point in order to separate, polymerize, and solidify the polymer network within the liquid crystal material.
More recently, in U.S. patent application Ser. No. 12/172,156 to Powers et al., thermotropic liquid crystal shutters have been described, wherein a thermotropic liquid crystal is placed between two crossed polarizers, such that in one temperature state the liquid crystal forms a twisted nematic waveblock that rotates the polarity of incoming light, allowing the light transmission, absorption, and reflection properties of a single polarizer, while in another temperature state the liquid crystal is in an isotropic state, such that it does not affect the polarization state of incoming light. The device has the optical properties of two crossed polarizers, allowing much lower transmission and much higher absorption or reflection of incident light. The information included in this Background section of the specification, including any references cited herein and any description or discussion thereof, is included for technical reference purposes only and is not to be regarded as subject matter by which the scope of the invention is to be bound.